Time delay control device



June 21, 1960 N. F. SCHUH, JR 2,942,123

TIME DELAY CONTROL DEVICE Filed Jan. 31, 1956 Fig. 2.

Voltage Fig. 3.

United States Patent M TIME DELAY CONTROL DEVICE Niles F. Schuh, Jr., Lima, Ohio, assignor to Westinghouse Electric Corporation, East Pittsburgh, Pa., a corporation of Pennsylvania Filed Jan. 31, 1956, Ser. No. 562,427 8 Claims. (Cl. 307-885) The present invention relates to time delay control devices, and more particularly to a time delay circuit for effecting operation of a relay or other control device with a predetermined time delay.

In the operation of control and protective devices in electrical systems, it is frequently necessary to provide a time delay, which may be either for a fixed time or for a time which varies inversely with the voltage to which the device responds. Various means have been used heretofore for obtaining time delay operation of relays and other control devices, but none of these time delay devices have been entirely satisfactory and they have serious disadvantages for many purposes.

Where a fixed time delay is required, the presently available devices, such as so-called slug type relays and thermal relays, are relatively unsatisfactory because they' have only a limited range of adjustment and because the time delay is not actually constant but is affected by temperature and applied voltage. In addition, thermal relays require a relatively large amount ofpower for operation, and require a cooling time after operation at least equal to the delay time. Both of these types of relays are limited in their range of adjustment, and there are certain ranges of time for which no satisfactory relay has been available.

In many cases a time delay is required which varies inversely with the applied voltage, as in over-voltage pro tective relays, for example, where rapid operation is required on high overvoltages but slower operation on lower voltages, to prevent undesired operation on transients. Various types of relays using dashpot arrange ments have commonly been used to obtain this type of operation. The induction disc type relay, in which a disc armature rotates at a speed proportional to the applied voltage to actuate the contacts, is also often used to obtain inverse time-voltage characteristics. Both of these types of relays obtain the desired inverse characteristic by means of moving mechanical parts. This is undesirable in many applications, however, such as in overvoltage relays for use in aircraft systems, where the relay is subjected to severe conditions of vibration, shock and acceleration forces, as well as to changes in mounting position and to wide temperature variations. The

2,942,123 Patented June 21, 196D tained which is very suitable for aircraft use because it can be made of small size and is highly reliable.

A further object of the invention is to provide a time delay control device or relay in which the control device is actuated by a static time delay circuit utilizing semiconductor devices and other static components of small size and high reliability to provide a time delay control device which is capable of adjustment over a wide range and which is not affected by mechanical shock or vibration or by other severe environmental conditions.

Other objects and advantages of the invention will be apparent from the following detailed description, taken in connection with the accompanying drawing, in which:

Figure 1 is a schematic diagram showing one embodiment of the invention;

Fig. 2 is a similar diagram showing the invention em bodied in an overvoltage protective relay; and

Fig. 3 is a diagram illustrating the operation of the circuit of Fig. 2.

Figure 1 shows the invention embodied in a time delay circuit which is energized =from a source of direct ourdesign of a relay which uses moving mechanical parts to capable of adjustment over a wide range of time and whichcan'readily be arranged to provide either a constant time delay or an inverse time-voltage characteristic.

Anotherobject of the invention is to provide a time delay' circuit made up of static devices of small size and highreliability, so that a completely static device is obrent voltage indicated by the lines 1 and 2. The lines 1 and 2 may be directly connected to a voltage to which the circuit is to be responsive, or they may be energized from such a voltage in any suitable manner, as by means of a rectifier from an alternating current voltage. The time delay circuit is preferably connected to the energizing voltage through a resistor 3 and includes a capacitor 4 connected across the energizing voltage in series with an adjustable resistor 5 and a semiconductor diode or rectifier device 6, preferably a silicon diode. The diode 6 is connected in the normally non-conducting direction, so that under normal conditions only ,a very small leakage current can flow to the capacitor.

Semiconductor diodes, such as silicon diodes, have a reverse characteristic which exhibits a very sharp breakdown voltage, often called the Zener voltage. That is, when a voltage is applied to such a diode in the reverse direction, only an extremely small leakage current flows through the diode until the voltage is increased to the breakdown value. At this point, the current suddenly increases to a value which is essentially limited only by the resistance of the circuit, while the voltage across the diode remains substantially constant. When the voltage is reduced below the breakdown value, the diode regains its rectifier characteristic and reduces the currentrto the very small leakage value. The circuit of the present in vention utilizes this characteristic of semiconductor diodes, and the diode 6 is used as a voltage-responsive device which prevents application of the energizing voltage to the capacitor 4 when the voltage is below a predetermined value and which applies the voltage to the capacitor when this value is exceeded. 'It will be understood, however, that any other voltage-responsive device, such as a relay, might be used to apply the energizing voltage to the capacitor when it exceeds the predetermined value.

The time delay circuit of Fig. ;1 is connected to actuate a relay 7 which has an operating coil 8 and contacts 9 which may be arranged in any desired manner. The relay 7 is actuated in response to the discharge current of the capacitor 4 when it has been charged by the energizing voltage to a predetermined level. For this purpose, the capacitor is provided'with a discharge circuit which includes semiconductor amplifier means, shown as a transistor 19. The capacitor 4 is connected to the base electrode 11 of the transistor through a resistor 12, and the emitter 13 of the transistor is connected to the other side of the capacitor 4 to complete the discharge circuit. T he collector 14 of the transistor is connected to the operating coil 8 of the relay 7, the other end of the coil being connected to the line 1. i A-second discharge circuit for the capacitor is also provided consisting of a resistor 15 and a rectifier 16, which may also be a silicon diode, connected in series across the capacitor, as shown, with the diode connected to normally permit current-flow from, the capacitor to the resistor. A- connection, 17 ispr'ovided' between the diodes 6. and 16V for the purpose of biasing the diode 16 under certain conditions, as explained below. The circuit of Fig. l is shown as being arranged to provide a constant time delay, and for this purpose another semiconductor diode 18. is connected across the line 1, 2, as shown, in the normally non-conducting direction. The diode 18 preferably has a breakdown voltagev slightly higher than that of the diode. 6.

In operation, when the energizing voltage isbelow the breakdown voltage of thediode. 6, the circuit is inactive and nosubstantial current ,fiows. in any part of. the circuit. The extremely. small. leakage current in the reverse direction through the diode 6 does not substantially charge the. capacitor. 4 because any charge immediately leaks ofi through the discharge circuit consisting of. the resistor 15,.

and diode 16, which isdesignedto have a low time constant so that no substantial charge can build up on the capacitor 4. If the voltage applied. to the circuit rises above the. breakdown voltage of the diode 6, the diode breaks down and reduces itsv resistance to a very small ductor17 by ahigher voltage than that of the capacitor,

softhat the capacitor cannot discharge through this circuit.

' As soon as the capacitor 4 startscharging, its voltage.

begins to rise and a discharge current flows through the resistor 12 to the base. 11, of. the transistor 10, the value of this current being limited by the resistor 12. When base current starts to. flow to the transistor, collector current also begins to. flow between the collector 14 and emitter 13 of the transistor; The. value ofv the collector current is, of course, determined by the base current and as. the. capacitor 4 continues, to. charge and increase its voltage, the base current increases and the collector current is correspondingly.increased. The relay coil 8 is energized by the collector current of the transistor and when this current reaches the required value, the relay 7 operates to actuate its contacts 9. Thus, in effect, the discharge: current of the capacitor 4 is amplified by the transistor to operate the relay 7. The diode. 18, as previously indicated, has a breakdown voltage slightly above thatof thediode 6, and the diode ISbreaks down immediately after. the diodev 6. The effect of this is to limit the voltage applied to the time delay circuit to a substantially constant value, any increase in voltage above the breakdown voltage of the diode-'18 appearing across the resistor 3. i t

The timev delay obtained by this circuit is determined by the rate of charging of the capacitor 4, which determines the time required for the capacitor voltage to reach the value necessary to produce the collector current required. to actuate the relay. In the circuit shown, the charging rate of the capacitor is determined only by the value of the resistor 5, since the voltage applied to the circuit is kept constant, and thus a constant time delay is obtained which is continuously adjustable over a wide range by adjustment of the. resistor 5. The fixed resistor 12 limits the discharge current of the capacitor and thus determines the value of capacitor voltage which is required to. produce the base current necessary to give the required collector current. By proper choice of the resistor 12, therefore, the circuit can be designed for any desired range of time delays.

It will be. apparent that a time delay circuit has been provided which is composed, entirely of: static devices of 'high reliability and. of small physical size. so that a small 4; and highly reliable device is Obtained. This circuit has many desirable characteristics; Thus, if the applied voltfaslls below the breakdown voltage of the diode 6 before the time delay period is completed, the diode 6 cuts off the current to the capacitor4 and the capacitor immediately discharges through the discharge circuit 15, so that no substantialcharge remains on the capacitor and the circuit is immediately ready for another operation without any efiect on the time delay. Similarly, after operation, any chargeremaining on the capacitor 4 is immediately discharged and the: circuit is ready for another operation. If an inverse time-voltage relation is desired, insteadv ofa. constant time, it is readily obtained merely by omitting the diode 18; If this is done, the charging rate of the capacitor. 4 is. determined both by the applied voltage and by the setting of the resistor 5, and

the voltage increases, the capacitor charges more rapidly so. that the time delay is decreased. The increase in voltage also tends to increase the; collector current of the transistor, for a given base current, thus causing the relay to operate sooner, although this is a relatively small factor.

Fig. 2 shows an embodiment of the invention in an overvoltage relay for a three-phase alternatingcurrent device to operate at a desired linevoltage. The time delay circuit 22' itself is arranged torinverse time-voltageoperation, but is otherwise generally similar to that of Fig. land includes a capacitor 25 connected across the energizing voltage in series with an adjustable resistor26 and a semiconductor diode 27 connected in the normally nonconducting direction, as shown. A discharge circuit for the capacitor 25, consisting of a resistor 28 and diode 29, is connected across the capacitor with a connection 30to the diode 27 for biasing the diode 29 duringoperation, as described. above, this discharge circuit preferably having a low time constant. The capacitor- 25 is also connected to another discharge circuit which includes another semiconductor diode 31, connected in thenormally non-conducting direction, and semiconductor amplifier means shown as a two-stage transistor amplifier including two transistors 2 and 33.

The relay 34 to be actuated by this circuit may be any suitable type of, relay, orother control device, having an operating coil 35 and contacts 36. The supply voltage for the relay 34 and the two-stage amplifier is obtained from the alternating cur-rent line 20 through a transformer 37 having its primary winding 38 connected across one phase of the line 20. The secondary winding 39 of the transformer is connected to supply a direct current output through a half-wave rectifier 40, a capacitor 41 being connected across the circuit to prevent chattering of the relay on the nonwonductive half cycles.

The base 42 of thetransis-tor 32 is connected to the capacitor 25 through the diode 31. The emitter 43 and collector 44 are connected to the base 45'of the second transistor 33, and to the transformer secondary winding 39, respectively, as shown. The emitter 46 of the second transistor 33 is connected to the capacitor 25 and to the other side of the transformer secondary winding, while the collector 47 of the transistor 33. is connected to the relay coil 35, the other end of the coil being connected to the, transformer winding.

The operationof this circuit is, similar to; that previously described. Thus, when the voltage of the 1ine 20 exceeds thefvalue for; which the device is set, the potentiometer' 24- appliesavoltage, to the. diode. 27 in excess of its breakdown voltage-t so that: urrent is p rmitted. to

The direct current voltage thus obtained through the resistor 26 to the capacitor to charge the capacitor. The diode 29 is biased by this voltage in the non-conductive direction so that the capacitor cannot discharge through the discharge circuit 2849, and the capacitor voltage increases at a rate determined by the setting of the resistor 26, and. the magnitude of the applied voltage, until it reaches the breakdown voltage of the diode 31. When the capacitor voltage reaches this value, the diode 31 breaks down and permits the capacitor to discharge to the base of the transistor 32. This discharge current is amplified by the two-stage transistor amplifier, and the output current of the amplifier, which is the collector current of the transistor 33, flows through the relay coil and actuates the relay 34 when the necessary value of this current is reached.

The operation of the circuit of Fig. 2 is further illustrated by the curves of Fig. 3. In this figure, voltage is plotted against time, and the line V represents the voltage applied to the time delay circuit 22. This voltage is represented as being zero, or at a value below the breakdown voltage of the diode 27, up to the point A at which time the voltage V, increases to a value in excess of the breakdown voltage of the diode 27. When current starts flowing to the capacitor 25, it begins to charge the capacitor and the capacitor voltage increases exponentially as shown by the curve V As the capacitor voltage increases, the breakdown voltage of the diode 31 is reached at the point B and current then starts flowing from the capacitor 25 to the base of the transistor 32. The base voltage of the transistors 32 and 33 increases along a curve such as V and the threshold value of the transistors 32 and 33 is reached at point C to permit collector current to flow from the transformer through the collector 44 and emitter 43 of the transistor 32 and through the base 45 and the emitter, 46 of the transistor 33 back to the transformer. At this point, collector current starts to flow from the transformer through the emitter 46, collector 47 and relay coil 35. The voltage across the relay coil starts rising along the curve marked V until the operating voltage of the relay is reached at point D, when the relay closes its contacts. Thus, as in Fig. 1, the capacitor discharge current is amplified and used to actuate the relay.

It will be seen from Fig. 3 that the total time delay is made up of the sum of the time delays from the point A to the point D, the time required to charge the capacitor being the greater part of the total period. As before, the rate of charging the capacitor is determined by the applied voltage and by the setting of the resistor 26, so that an inverse time voltage characteristic is obtained and the delay is continuously adjustable over a wide range by adjustment of the resistor 26. The curves of Fig. 3 also illustrate another important advantage of the invention, since it will be seen that the relay coil is energized for only a very small part of the total time delay period, so that there is no danger of the relay being actuated prematurely by physical-shock applied to the relay when it is partly energized, since it is not energized at all until almost the end of the delay period.

It will now be apparent that a time delay circuit has been provided which has many advantages. In either of the embodiments of the invention shown, only static components of small size and high reliability are utilized, so

that a small and rugged device is obtained which is not easily damaged and which is not substantially affected by environmental conditions or by mechanical shock or vibration, and the device is very suitable for aircraft use, and similar applications, where small size and high reliability are essential. A rugged and small direct current relay may be used with this circuit or,'if desired, the relay could be replaced by a static bistable circuit of any suitable type, resulting in a completely static control device. The circuit is continuously adjustable over a wide range to obtain any desired time delay and it can readily be adapted for either'a fixed time delay or for inverse 6 time-voltage characteristics in the manner described above. I

It will be obvious that various changes and modifications may be made within the scope of the invention. Thus, the circuit may be used to actuate any type of control device, and is not limited to the particular application shown. The resistors 5 and 26 may be replaced by a non-linear resistor, if desired, in order to obtain any desired time-voltage characteristic, a selenium rectifier or a series-connected group of selenium rectifiers being a very suitable type of non-linear resistor for this purpose. Similarly, the transistors utilized in the circuits shown are indicated as being of the N-P-N type, but it will be obvious that transistors of the P-N-P type could be used equally well, with a suitable change in the polarities of the other components of the circuit. It is to be understood, therefore, that'although certain specific embodiments of the invention have been shown and described for the purpose of illustration, it is not limited to these particular embodiments, but includes all equivalent modifications and embodiments.

I claim as my invention:

1. A time delay circuit comprising a capacitor, means for applying to the capacitor a voltage to which the circuit is to respond, resistance means for controlling the rate at which the capacitor is charged by said voltage, a transistor connected to provide an output current, means for connecting the capacitor to said transistor to discharge therethrough to control the output current of the transistor, said connecting means including means for controlling the capacitor discharge current, and a second discharge circuit connected across the capacitor, said second discharge circuit including rectifier means connected to prevent current flow when said voltage is applied to the capacitor. 2. A time delay circuit for effecting actuation of a control device, said circuit comprising a capacitor, means for applying to said capacitor a voltage to which the circuit is to respond, resistance means for controlling the rate at which the capacitor is charged by said voltage, a transistor connected to provide an output current to effect actuation of the control device, and means for connecting the capacitor to said transistor to discharge therethrough to control the output current of the transistor, said con necting means including means for preventing effective flow of current from the capacitor until the capacitor voltage exceeds a predetermined value.

3. A time delay circuit for effecting actuation of a control device, said circuit comprising a capacitor, means for applying to said capacitor a voltage to which the circuit is to respond, resistance means for controlling the rate at which the capacitor is charged by said voltage, a transistor connected to provide an output current to effect actuation of the control device, and means for con necting the capacitor to said transistor to discharge there through to control the output current of the transistor, said connecting means including a semiconductor diode connected to prevent current flow from the capacitor until the capacitor voltage reaches a predetermined value.

4. A time delay circuit for effecting actuation of a control device, said circuit comprising a capacitor, means for applying to said capacitor a voltage to which the circuit is to respond, resistance means for controlling the rate at which the capacitor is charged by said voltage, a discharge circuit for the capacitor including transistor amplifier means for producing an output current determined by the capacitor discharge current to effect actuation of the control device, means for controlling the capacitor discharge current, and a second discharge circuit connected across the capacitor, said second discharge circuit including rectifier means connected to prevent current flow when said voltage is applied to the capacitor.

5. A time delay circuit for efiecting actuation of a control device, said circuit comprising a capacitor, means for applying to said capacitor a voltage to which the circuit is to respond, resistance means for controlling the rate at which the capacitor is charged by said voltage, a discharge circuit for the capacitor including transistor amplifier means for producing an output current determined by the capacitor discharge current to efiect actuation of the control device, said discharge circuit including means for preventing efiective flow of current until the capacitor voltage exceeds a predetermined value, and a second discharge circuit connected across the capacitor, said second discharge circuit including rectifier means connected to prevent current flow when said voltage is applied to the capacitor.

6. A time delay circuit for effecting actuation of a control device, said circuit comprising a capacitor, means for applying to said capacitor a voltage to which the circuit is to respond, resistance means for controlling the rate at which the capacitor is charged by said voltage, a transistor connected to provide an output current to eifect actuation of the control device, means for connecting the capacitor to said transistor to discharge therethrough to control the output current of the transistor, said connecting means including a semiconductor diode connected to prevent current flow from the capacitor until the capacitor voltage reaches a predetermined value, and a second discharge circuit connected across the capacitor, said second discharge circuit including rectifier means connected to prevent current flow when said voltage is applied to-the capacitor.

7. A time delay circuitfor effecting actuation of a control device, said circuit comprising a capacitor, a semiconductor diode connected to apply to the capacitor a voltage to which the circuit is to respond when said voltage exceeds a predetermined value, adjustable resistance means for controlling the rate at which the capacitor is charged by said voltage, a transistor connected to provide an output current to effect actuation of the control device, means for connecting the capacitor to ceeds a predetermined value, means for controlling the rate at which the capacitor is charged by said voltage, circuit means for permitting the capacitor to discharge, said circuit means including means for preventing current flow from the capacitor until the capacitor voltage reaches a predetermined value, and a second discharge circuit connected across the capacitor and including rectifier means connected to prevent current flow when the first-mentioned voltage is applied to the capacitor.

References Cited in the file of this patent UNITED STATES PATENTS 2,020,727 Gardes et al. Nov. 12, 1935 2,427,751 Snyder Sept. 23, 1947 2,453,978 Elmendorf Nov. 16, 1948 2,584,990 Dimond Feb. 12, 1952 2,589,085 Houghton Mar. 11, 1952 2,609,506 Siezen Sept. 2, 1952 2,774,888 Trousdale Dec. 18, 1956 2,824,287 Green et al Feb. 18, 1958 OTHER REFERENCES Radio and Television News, vol. 50, iss. #4, pp. 68, 69, 187, October 1953. 

